I am creating a development tool for web development. This tool loads a users webpage into an iframe, this allows the program to resize the iframe and simulate mobile screen sizes. I also want to build in a tool that automatically refreshes the iframe when local content is changed. To do this I am going to use Ajax to load files in and every 5 seconds or so compare the file to its previous version from 5 seconds ago. I currently have it working with just one file.
If you want to access the client's disk file system then look into an embedded client application which you serve up from your webpage, like an Applet, Silverlight or something like that. If you like to access the server's disk file system, then look for the solution in the server side corner using a server side programming language like Java, PHP, etc, whatever your webserver is currently using/supporting.